1. Field of Invention
The present invention relates to the field of polyurethane foams. More particularly, it relates to polyurethane foams having improved slip resistance.
2. Background of the Art
It is well known that polyurethane foams may be obtained by the reaction of an isocyanate component with an active hydrogen-containing component, such as, for example, a polyether or polyester polyol, and, optionally, a blowing agent. For processing convenience it is, in certain circumstances, advantageous to use, as the isocyanate component, an isocyanate-terminated prepolymer. Such prepolymer may be prepared by, for example, the reaction of an excess of an aromatic diisocyanate with a relatively low molecular weight diol such as, for example, dipropylene glycol and/or tripropylene glycol. The blowing agent may be an inert physical blowing agent, such as, for example, a chlorofluoro compound, and/or a reactive blowing agent, such as, for example, water. The preparation of polyurethane foams by such procedures is described in, for example, patent publications EP 235,888; EP 175,733; and U.S. Pat. Nos. 3,591,532; 3,901,959; 4,647,596; and 4,757,095.
Current manufacturing trends for polyurethane foams are driven by two dominant factors, namely, cost and environmental concern. Consideration of cost and superior humid ageing performance leads polyurethane foam producers to select polyether polyols over polyester polyols, while consideration of environmental issues leads such producers to select water in preference to certain physical blowing agents. However, use of water as a blowing agent may compromise certain properties, such as low temperature durability, hardness and processability. Some of these problems may be mitigated by using an isocyanate-terminated prepolymer prepared by reaction of an excess of an aromatic diisocyanate with a relatively high molecular weight polyether diol or triol.
One popular use for polyurethanes currently is in the manufacture of shoesoles. In this application polyurethanes offer versatility, processability, comfort and durability. However, some polyurethane formulations, particularly those based on polyethers, do not offer acceptable or desirable levels of slip resistance. This problem is particularly problematic when the surface of the shoe sole will contact “wet” conditions, such as contact surfaces where water, aqueous solutions, grease, and/or oil are present, all of which tend to actually lubricate the contact point(s) which can result in a slip or fall. An acceptable level of slip resistance is needed to reduce or prevent slipping or falling accidents.
In view of this need, objective standards have been developed to measure slip resistance of a polyurethane material in contact with both dry and wet surfaces. It is measured in terms of the coefficient of friction (COF), also known as frictional coefficient or friction coefficient, between the surface of the polyurethane being tested and a given test surface, such as steel or a pressed ceramic floor, under given test conditions, such as in the presence of glycerol or a detergent solution. It is symbolized by the Greek letter μ and is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. Friction is defined as the resistance to motion that occurs whenever one solid body slides over another.
One particular test standard is specified in the SATRA Technology Centre's test protocol TM144 and in the International Organization for Standardization's UNI EN ISO 13287:2006 standard. This European standard specifies a method for testing the slip resistance of conventionally soled safety, protective and occupational footwear, and is not applicable to special purpose footwear containing spikes, metal studs or similar anti-slip devices. It is well-known and generally accepted that a minimum COF value of around 0.30 is necessary to prevent most wearers from slipping during normal walking. For activities that are more dynamic, such as running, higher COF values are strongly recommended.